This is a proposal to study the synaptic organization of the PMBSF region of mouse somatosensory cortex. This region of cortex is composed of functional columnar units which are the somatotopically arranged cortical representation of a peripheral receptor system - the mystacial vibrissae. The specific objectives of the proposed studies are (1) to identify neurons postsynaptic to thalamocortical axon terminals, (2) to identify those cortical neuronal types postsynaptic to the axon terminals of neurons which, themselves, synapse with thalamocortical afferents, and (3) to identify neurons which synapse with axon terminals whose somata are located in ipsilateral SmII and MsI cortex. An axonal tracing technique using lesion induced degeneration, and electron microscopy of serial thin sections, will be employed to identify axon terminals from sources extrinsic to PMBSF cortex, i.e., from the thalamus and from ipsilateral cortical areas SmII and MsI. Golgi impregnated neurons identified with the light microscope will be processed by a chemical procedure which replaces the dense Golgi precipitate with a deposit of fine gold particles. The "deimpregnated" neurons are still visible with the light microscope because of their content of gold; in the electron microscope the gold particles are so distributed that the cytology and synaptology of the deimpregnated neurons can be determined. This method will be used to identify cortical neuronal types postsynaptic to degenerating axon terminals from the thalamus or from ipsilateral cortical areas SmII or MsI. The Golgi/EM deimpregnation method will also be used to trace the intrinsic axonal projections of deimpregnated neurons known to receive input from the thalamus. Elements postsynaptic to these deimpregnated axon terminals will be identified by reconstructing them from series of thin sections. It might also be possible to identify these elements by comparing their cytology and synaptology with those of neuronal elements which have been identified by the Golgi/EM method. The proposed studies are designed to yield specific information about the extrinsic synaptic input to, and intrinsic synaptic connections of identified neurons in mammalian neocortex.